How do I save a certain workspace as the default workspace (not default upon opening blender)
Any changes you make to any of the workspaces can be saved as part of your Startup File.
Here's my Compositing screen, I also enabled the denoising data passes so I could denoise renders by default.

Just open a new file, make any changes, and go to this menu.  Whatever workspace you're currently in will be the one Blender opens to.